Cost and risk assessment for spacecraft operation decisions caused

Acta Astronautica 113 (2015) 66–79
Contents lists available at ScienceDirect
Acta Astronautica
journal homepage: www.elsevier.com/locate/actaastro
Cost and risk assessment for spacecraft operation decisions
caused by the space debris environment$
Hanspeter Schaub a,n, Lee E.Z. Jasper a, Paul V. Anderson a, Darren S. McKnight b
a
b
University of Colorado, Boulder, CO 80309, USA
Integrity Applications Incorporated, USA
a r t i c l e in f o
abstract
Article history:
Received 12 October 2014
Received in revised form
23 February 2015
Accepted 27 March 2015
Available online 4 April 2015
Space debris is a topic of concern among many in the space community. Most forecasting
analyses look centuries into the future to attempt to predict how severe debris densities
and fluxes will become in orbit regimes of interest. Conversely, space operators currently
do not treat space debris as a major mission hazard. This survey paper outlines the range
of cost and risk evaluations a space operator must consider when determining a debrisrelated response. Beyond the typical direct costs of performing an avoidance maneuver,
the total cost including indirect costs, political costs and space environmental costs are
discussed. The weights on these costs can vary drastically across mission types and orbit
regimes flown. The operator response options during a mission are grouped into four
categories: no action, perform debris dodging, follow stricter mitigation, and employ ADR.
Current space operations are only considering the no action and debris dodging options,
but increasing debris risk will eventually force the stricter mitigation and ADR options.
Debris response equilibria where debris-related risks and costs settle on a steady-state
solution are hypothesized.
& 2015 IAA. Published by Elsevier Ltd. All rights reserved.
Keywords:
Space debris
Debris mitigation
1. Introduction
The presence and creation of debris due to human
operations in orbit is an ongoing problem. It is recognized
that the continuation of current trends in launches and long
orbital lifetimes of satellites will only increase the density of
debris in both Low Earth Orbit (LEO) and High Earth Orbit
(HEO) regimes, such as geosynchronous (GEO) [1–4]. This has
led to increased use of passivation techniques to avoid onorbit break-ups, improved spacecraft shielding against small
object impacts, and the mitigation guidelines of a 25-year
lifetime rule for LEO and sub- or super-synchronous
☆
This paper was presented during the 65th IAC in Toronto.
Corresponding author.
E-mail addresses: [email protected] (H. Schaub),
[email protected] (L.E.Z. Jasper),
[email protected] (P.V. Anderson),
[email protected] (D.S. McKnight).
n
http://dx.doi.org/10.1016/j.actaastro.2015.03.028
0094-5765/& 2015 IAA. Published by Elsevier Ltd. All rights reserved.
graveyard orbit for GEO. Active Debris Removal (ADR) has
also been suggested, and widely studied, as a possible
method for reducing debris density. However, ADR techniques considered in the literature, such as robotic re-orbiting
[5–8], electrodynamic tethers [9,10], laser ablation [11–14],
ion shepherd methods [15–18], tethered tugging of large LEO
debris [7,19–24], harpoons or nets to capture debris [7,25,26],
and electrostatic tractors [27–30], are economically costly,
technically challenging to develop, and often overshadowed
by political hurdles [31,32]. More recently, Just-in-time Collision Avoidance (JCA) concepts are discussed where the orbit
of a large debris object is nudged with an intercept mission to
avoid collisions with operating assets or other debris objects
[33]. Such technology could be more cost effective than ADR,
but requires highly accurate debris tracking and leaves the
debris in orbit.
There are many important research papers discussing
the projected growth of space debris in the near Earth
environment, such as the often cited studies by Liou
H. Schaub et al. / Acta Astronautica 113 (2015) 66–79
published in References [1] and [2]. Here, the LEO debris
population greater than 10 cm in size is modeled for the
next 100–200 years, showing that even with an optimistic
50% mitigation compliance rate, the LEO debris population
could double over 200 years. Reference [34] shows the
debris doubling over 100 years if no mitigation methods
are implemented. While such figures are alarming to space
debris researchers and experts who understand that these
results represent mean trends, the worst-case scenarios
could be much more severe. Convincing the general public,
policy makers, and research funding agencies that action is
required now to control this debris hazard remains a
challenge. For example, operators today are able to fly
satellites in their desired LEO or HEO orbits with only
minimal concern regarding space debris avoidance. When
asking unmanned satellite operators how often they need
to make an additional maneuver to avoid debris, the
common answer is that this almost never happens. If
there is a warning of a possible conjunction, the uncertainty of the miss distance is often so large that the
warning is ignored, or the conjunction is accounted for
in regular orbit maintenance maneuvers, thus not expending additional fuel. Therefore, considering that space
debris strikes have had a minimal documented impact
on current satellite operations, doubling or tripling debrisrelated risk – especially 100–200 years in the future – is
unlikely to convince policy makers or operators to demand
strong space debris mitigation and remediation policies
over the next decade. There are significant on-going efforts
to better attribute many anomalies and failures of
unknown cause to their trigger.1 some of these anomalies
may have been caused by non-trackable debris, the true
current space debris threat has not yet been captured or
communicated.
Reference [35] discusses the need to consider nearterm ADR (remediation) developments and stronger endof-life disposal guidelines (mitigation). The complexity of
considering LEO space debris risks is shown by how the
fragment sizes and orbit types impact the risk to the space
operator. Vance proposes in Reference [36] an economic
metric by which competing debris removal methods are
evaluated for the highly populated sun-synchronous orbit
regime. However, this orbit-specific analysis only considers cost due to the economic value of the satellite, and the
environmental cost if the satellite experiences a fragmentation collision. Risk costs of the de-orbit maneuver, costs
incurred by precision tracking of the debris to be removed,
and political cost considerations are not included.
Thus, this paper investigates a means to bridge the
divide between space debris researchers that support
near-term action (begin ADR within a decade) to control
the space debris population, and most space operators that
are successfully operating satellites without demanding
stronger mitigation and remediation methods. In particular, this study highlights the complex decision logic that
space operators face when considering the total space
debris-related cost. The available debris response options
1
http://www.integrity-apps.com/events/scaf/.
67
during a mission are classified under one of the following
categories:
1. Make no mission changes in response to space debris.
2. Respond to conjunction warnings by dodging closeapproach debris or using JCA.
3. Follow current or more stringent end-of-mission mitigation guidelines.
4. Begin active debris removal or remediation in the orbit
regime of interest.
Currently only elements of options 1 or 2 are employed
in the operator community. Implementing shorter postmission orbital lifetimes (element of option 3) can have a
significant impact on the commercial viability of launch
operation if it is not uniformly adopted. Elements of option
4 are discussed and researched, but economically viable
and proven solutions are at least a decade away from being
flight ready. The natural question arises: At what point is
the total space-debris-related cost large enough to warrant
options 3 or 4? This paper considers a high-level decision
logic from an operator's point of view on how to respond
to a space debris threat including not only direct missionrelated financial considerations, but also indirect costs
such as tracking or debris avoidance analysis, environmental and political considerations. While earlier studies
focus on the overall space debris growth, the impact to the
individual space operator can vary by orders of magnitude
depending on where the satellite is flown, the mission
duration, and the mission objectives (e.g., high-value
commercial communication satellite versus low-cost
CubeSat technology demonstration).
The paper outline is as follows. First, the present-day
status of the LEO and GEO debris environment is reviewed.
Next, the overall space debris costs and associated
response decision factors are discussed, illustrating how
these can vary drastically across mission types. A mission
scenario case study illustrates how different mission types
are impacted very differently by space debris, leading to
the current range of operator responses to debris-related
risk. This is important when trying to bridge the divide
between space debris researchers and operators/policy
makers. A fundamental question is asking whether common mitigation guidelines for all LEO operators make
sense. Another important aspect to consider is what
happens if stronger mitigation or ADR measures are
implemented. In particular, would these ADR efforts continue indefinitely, or could the debris control methods
stabilize to new operational equilibriums? Finally, the
possible operator responses and costs to the debris threat
are reviewed and discussed.
2. Present day space debris congestion
LEO is the most studied orbit regime for orbital debris –
this is because it is the most densely populated regime (using
spherical shell densities), as illustrated in Fig. 1, and many
commercial, government, and military satellites are in this
regime. GEO has the next largest spherical density, while
Oltrogge states that its volume density can be as critical as
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H. Schaub et al. / Acta Astronautica 113 (2015) 66–79
Fig. 1. Density of debris population from LEO to GEO regimes, reproduced from Reference [37], compared to insurance values provided in
Reference [38].
LEO [39]. GEO is critically important for commerce and Earth
observing, with pivotal assets existing for many organizations.
This is illustrated in Fig. 1 by comparing the debris spatial
densities to the orbit regime insurance values discussed by
SwissRe in Reference [38].2 95% of insured satellites systems
reside in GEO, and such costs must be considered when
evaluating the economic impact if a satellite is struck by
debris. Although this paper focuses on the LEO and GEO
regions, the issues, risks, and costs associated with debris
relate to other orbital regimes as well.
Fig. 2 illustrates the altitudes and inclinations of LEO
rocket bodies and spacecraft with masses above 50 kg [34].
Note that these objects are not evenly spread out over the
orbit altitudes and planes; rather, distinct banding is
observed. This illustrates that the LEO risks are focused
in select orbit regimes. The sun-synchronous region is
illustrated as it has a very high economic value for
commercial remote sensing and imaging satellites, as well
for some military reconnaissance systems [38]. This region
also has the highest debris collision hazard – the annual
probability of collision of a 10 m2 satellite with 1 cm debris
or larger exceeds 0.8% [38].
There are several excellent studies of the LEO debris
environment, many emanating from NASA's Johnson Orbital Debris Program Office and ESA's Space Debris Office
[1,2,34,40–43]. Reference [44] highlights the variability in
these debris forecast studies. For brevity, the following can
be summarized from these studies:
longitude-dependent congestion levels across the GEO
ring [46,47]. In addition to this large-scale, catalogued
debris population, significant populations of uncatalogued
objects at sizes as small as 10–15 cm have been detected in
GEO optical observation campaigns, and are hypothesized
to be indicative of undetected fragmentation events in this
regime [38,48,49]. Recent studies of the GEO environment
illustrate that GEO debris congestion—and the resulting
probability of collision—is non-uniform in longitude and
time (both time of day and time of year) [50,51].
Fig. 3 depicts the number of longitude-dependent nearmiss events per day within 50 km for a projected five-year
forecast, assuming an idealized “no future launches”
scenario. While 50 km may appear like a large distance,
larger than the typical GEO orbit determination accuracy,
the high value of GEO satellites has many operators
studying objects at even larger distances. As shown in
Fig. 3, longitude slots in the vicinity of the two gravitational wells at 75E and 105W are subject to upwards of 5–
6 close calls per day with uncontrolled debris objects, in
contrast to the longitude slots over the Atlantic and Pacific
Oceans, which experience a maximum of only 1–2 close
calls per day at this miss distance. Longitude-dependent
debris congestion patterns, and alternative debris flux
descriptions that provide higher spatial and temporal
resolutions [51], are central to mission assurance and
space situational awareness activities in the GEO ring,
and have critical implications for both the direct and
indirect costs incurred by operating a satellite in the GEO
debris environment.
An open question regarding space debris is, what is a
sustainable debris environment [52]? The studies to date
consider the effort required to keep the debris at current
levels. This is true for both pre- and post-2009 studies. The
acceptable debris environment simply shifted after 2009.
However, is the current space debris level required for
sustainable space operations? Could the debris grow 50%, or
double, while still keeping space operations economically
viable? This important question is not addressed in this paper,
but must be considered when doing a cost/benefit analysis of
debris mitigation and remediations efforts.
3. Debris considerations for operators
1. The number of debris is increasing, and appears to have
rapid growth in certain altitudes [4,41].
2. High inclination orbits are most densely populated,
especially around 600–800 km and 1000–1500 km altitude [34,40].
3. Large debris objects drive growth, but smaller objects
(0.5–10 cm) pose the largest hazard to active satellites
because they are numerous, have significant energy,
and cannot be tracked [45].
As of February 2014, the GEO regime contains approximately 1145 large-scale, unclassified, and trackable objects
larger than 0.8–1.0 m in effective diameter, 760 of which
are uncontrolled derelict objects that actively contribute to
2
The Technical Report is available at http://media.swissre.com/docu
ments/Publ11_Space þ debris.pdf.
3.1. Operator debris response considerations
Fig. 4 outlines the high-level weighted cost considerations that a satellite operator must consider to determine
how to respond to the orbital debris environment threat.
In this discussion, the term “cost” does not relate to
monetary objects exclusively, but can relate to political
implications and mission risks, as well. Thus, the cost
structure shown in Fig. 4 provides a total debris-related
cost function that considers a heterogenous set of “debris
costs”, each weighted considering the associated orbit
regime and satellite type. The mission-related responses
(right side) are grouped into four categories. The first two
are short-term reactive responses to an immediate debris
threat during a mission and include: (1) no action is made
during a mission in response to debris, and (2) a trackable
debris threat is reduced through a maneuver. The next two
H. Schaub et al. / Acta Astronautica 113 (2015) 66–79
Thrusting: Considers the cost of having to implement
thrusting. This would have a strong weight on
small satellites that typically don't have thrusting, or very little fuel. On most other bodies this
weight is low as these satellites already have
thrusting capabilities. De-orbiting rocket bodies
after they deliver their payload would require a
lot of fuel, thus the strong weight.
Downtime: Considers the impact if mission operations
have to be suspended to address debris issues.
This has a strong weight for commercial or
military satellites.
Insurance: Space insurance costs are heavily weighted
towards commercial GEO satellites. Small satellite operators don't often purchase insurance.
While the US government satellites don't typically carry insurance, some other countries' governments do purchase satellite insurance.
Mitigation: Considers the costs to implement current
mitigation guidelines. This is a large challenge
for CubeSats and small satellites, and limits them
often to lower altitudes. For commercial launch
providers, the cost of de-orbiting their spent
Month/
Mon
h/Year
ear
Fig. 2. Illustration of the LEO debris hazard [34] and regions of high
economic interest.
rocket bodies has a significant impact on their
competitiveness.
Debris tracking: This is mostly covered by the US Air Force,
but other countries are starting to develop their
own space situational awareness programs [53].
Debris analysis: Considers the labor time and costs of
analyzing possible debris threats.
Risk of de-orbit: Considers the risk of debris causing
damage on Earth. This is negligible for small
satellites, strong for LEO satellites, and has no
weight on GEO satellites.
Reputation Loss: Considers the political and professional
impact if the operator's satellite causes damage
to another satellite.
Space environment: Considers the risk either operating in
a high-density debris environment, or the risk to
other operators if the operator's satellite causes a
large debris field.
Currently the practice is to either not consider space
debris, or account for possible conjunction events in their
regular orbit correction maneuver planning, as illustrated in
Fig. 5. Prior to the Iridium/Cosmos collision in 2009, the
response was mostly to do nothing. Even now, the spacecraft
operators and insurance industry do not appear overly
concerned with addressing space debris. Malfunctions
known to be related to debris are currently still rare [54].
In contrast, space debris research is strongly arguing for
increased space debris mitigation or remediation. Operators
of low-risk assets with short mission lifetimes are less
concerned with debris and therefore typically choose the
no action response. As total debris hazard related risk
increases, it is necessary to maneuver through the debris
field, or increase the spacecraft shielding, increasing all
related costs and risks. These approaches are reactive since
operators are only responding to the current debris environment. As the debris-related risks rise to a critical level, likely
driven by major catastrophic events such as collisions, the
total costs will justify pursing stricter mitigation than the
current IADC standards. If the environment becomes significantly more hostile to space operators, ADR may be
required, thereby increasing the costs to remediate the
debris environment considerably. These latter two
approaches actively attempt to alter the environment, providing a proactive approach to risk reduction.
Current technology status makes mitigation costly and
remediation all but unaffordable – this leads to a strong
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03/ 17
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03/ 16
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03/ 15
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0
30E
60E
90E
120E 150E 180 150W 120W 90W 60W 30W
Long
ngitude [de
deg]
Fig. 3. Forecasted longitude-dependent debris congestion in GEO regime.
0
0
Near-M
Nea
-Miss Eve
vents [50 km]
m]
are proactive, long-term responses that seek to avoid the
possible creation of future debris including: (3) a following
of stricter mitigation guidelines, and (4) using ADR to
reduce the long-term debris in an orbit regime of interest.
The multitude of debris-related costs a space operator
must consider are discussed in detail throughout the
remainder of this Section. Interestingly, the answers to
the questions in Fig. 4 can vary strongly from one operator
to another as illustrated in Table 1. The table columns
indicate the following considerations:
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H. Schaub et al. / Acta Astronautica 113 (2015) 66–79
Fig. 4. Illustration of the cost considerations leading to a space debris response decision.
Table 1
Representative weighting for various debris hazard impacts on mission types using the coloring.
– strong,
– medium,
– low and
– no
weight
Short-Term (Reactive)
Long-Term (Proactive)
Level of Perceived Total COST
Pre-2009
Today?
No Action
Debris
Dodging
Stricter
Mitigation
Enact ADR
time. Seeking ADR technologies that have a broader
impact will facilitate their development, and spread out
the costs over a range of mission operations such as orbital
servicing, or autonomous refueling. The current space
debris mitigation requirements are a step in the right
direction, but do they go far enough? Instead of worrying
about active removal, or increased costs of tracking an
ever-growing debris population, what are the costs associated with requiring a greatly reduced post-mission
satellite disposal time (e.g., proposing a 10-year reentry
rule for certain LEO satellites, instead of the current
general 25-year standard)?
3.2. Direct costs
Level of Perceived RISK
Fig. 5. Trend showing that as perceived debris-related risk increases, so
does associated operator costs.
uncertainty in this debris response cost function. As outlined by McKnight in Reference [35], such advanced
technology development can take many years, even a
decade. Further, the funds required to achieve the technology readiness level for ADR increase dramatically as the
development time is decreased. Thus, there is a large risk
associated with waiting until the daily operator costs
associated with space debris are large, and then seeking
a mitigation/remediation solution over a short period of
There are a multitude of direct costs associated with
operating in the debris environment around Earth. A direct
cost is considered a cost associated with a particular launch,
satellite, or operation of a specific satellite. These values can
change dramatically across ranges of mission types.
While the “no action” during a mission approach costs
nothing for operators, there are often related direct costs
with increased satellite shielding. Such protection provides
the operator with a risk reduction relative to the lethal, yet
Non-Trackable (LNT) debris objects. Many of the larger, more
capable satellites consider maneuvers or “dodging” debris. A
strong concern with performing debris avoidance maneuvers
is the fuel costs and the possible satellite down-time,
H. Schaub et al. / Acta Astronautica 113 (2015) 66–79
reducing science return and revenue. The satellite downtime can be a stronger cost to commercial operators than the
fuel usage. For smaller science satellites that only have a
small amount of fuel aboard, it can be more costly to
consume fuel, resulting in significantly shorter mission lifetime as illustrated in Table 1.
Orbital debris has built up over the years because it is
less costly to abandon the spacecraft than choosing “mitigation” or post mission disposals. It is directly expensive to
operators and launchers to perform mitigation operations.
Even for government payloads, contracts are often
awarded based upon lowest price, thus it is more competitive to not have mitigation costs. In a study by Adilov
et al. [55] they demonstrate that choosing to create debris
leads to less overall direct cost to operators, and higher
profits. They further point out that, for commercial applications, it is more competitive to have more operational
assets than necessary. This means that it is, again, more
profitable to create debris. If an asset is lost, it will be
replaced adding further mass and objects in orbit, increasing debris growth. While this description is based upon a
simple economic model, this is supported by actual practices of companies that have reduced mitigation practices
due to their expense [56].
Spacecraft insurance is another direct cost to be considered. First-party policies insure against the failure of the
asset, while third-party policies covers satellite owners for
suits that may be filed by third parties in the event that
their satellite hits another satellite and damages that other
satellite.3 While many of the smaller satellite operators
don't carry insurance, the larger commercial, and some
non-US government satellites, do carry first party insurance with a world wide combined insurance premium cost
of about $800 million per year. Third-party insurance is
still rarer, only larger commercial satellites in LEO and GEO
carry it, with the world-wide insurance premiums summing to about $20 million per year (see footnote 1).
However, though space debris rarely factors into the current
insurance cost premiums, insurance companies are beginning
to consider debris [38,57]. The worst LEO debris-related mission risk is about 0.8% [38], while the total on-orbit failure risk
is about 1.5%. Rocket bodies, after having delivered their payload to orbit, may still be covered by third party insurance for
30 days to one year. Thus, rocket body operators debris cost
concerns don't end with the mission termination. For thirdparty spacecraft insurance the premium would be lower if
there is a way to avoid a collision as the insurance risk is
dominated by collision concerns. With first-party insurance the
concerns are dominated by mechanical breakdown issues with
the satellite itself, rather than with debris-related collisions. The
incremental risk is small enough to be ignored for now (see
footnote 1). However, this could rapidly change if there is a
catastrophic collision of a large, insured satellite. Thierry Colliot
is quoted as saying “You can potentially lose the premium of a
whole year in one single event” [58]. With the increased
liability of the launch vehicles, satellite insurance rates have
been dropping, leaving very thin margins and higher risk if a
3
Information obtained in conversation with Chris Kunstadter.
71
failure would occur.4 The insurance costs illustrate how the
space debris costs are driven by outlier event.
3.3. Indirect costs
An indirect cost is one that occurs due to generally having
debris in orbit, but is not unique to the particular mission.
Examples include the cost of tracking debris, or staffing for
debris-related analysis. Unfortunately, the natural progress
towards creating more orbital debris distributes indirect costs
to all entities that utilize satellites. Perhaps one of the largest
costs, that has yet to be well understood, is the cost to the US
Air Force (USAF). While the USAF tracks all visible objects in
orbit for its own Space Situational Awareness (SSA) defense
reasons, the creation of debris further stresses this organization. This affects operators because the USAF freely provides
tracking, conjunction, and some Collision Avoidance (COLA)
analysis for most satellite operators today [59]. With the
Iridium–Cosmos collision, the collaboration between the USAF
and operators around the world has increased. Still, the USAF
is not obligated to provide information to operators and these
indirect costs get distributed to US citizens. The community is
currently dependent upon US government cooperation.
Implementation of more precise and smaller debris tracking
is a costly undertaking. Several nations are investigating
improving their own space situational awareness capabilities
[53]. The US government accountability office May 2011
report [60] outlines the significant fiscal and managerial
challenges of expanding the SSA capabilities. Space debris
growth is mentioned as one of the drivers for needing
improved SSA. The US SSA-related investments over 2006–
2015 sum up to about 5.3 billion US dollars. This does not
include the costs of operating the existing SSA programs and
facilities. However, factoring out the specific debris-related
cost components is very challenging as many of these SSA
systems have non-debris-related functions as well.
The analysis of a conjunction, often automated, and
maneuver planning can be considered indirect costs to the
mission operator [61]. Conversations with DigitalGlobe
indicate that the cost of monitoring JSpOC warnings,
analyzing them, and occasionally planning COLA maneuvers (less than 10 per year), is about 15% of the cost of one
full time engineer. The creation of the automated analysis
system costs more, but it was a one-time cost for the
company. Further, COLA maneuvers do not reduce lifetime
of the spacecraft at this point because they can be built
into the normal station keeping maneuvers. Assuming
DigitalGlobe is similar to many other LEO operators, it is
a tangible, but minimal, indirect cost to operate in the
current debris environment. This is the major reason why
there are strongly conflicting responses to the current
debris environment between operators (little to no
response) and the space debris research community.
3.4. Political costs
There are five United Nations treaties on outer space.
For example, the 1972 Liability Convention5 specifies that
4
http://tinyurl.com/pmr53n9.
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H. Schaub et al. / Acta Astronautica 113 (2015) 66–79
nations are internationally responsible for in-space or onEarth damage caused by their space objects [62]. The 1975
Registration Convention6 declares that states will register
and report the launch of space assets. Art. VIII of the 1967
Outer Space Treaty7 provides that States keep under their
jurisdiction the space objects that they have registered.
Unfortunately, there is no clear legal obligation for various
states to exchange information to avoid collisions. Thus,
except for specific bi-lateral agreements, the US Air Force
can stop its dissemination of information at any time. Articles
II, III and IV of the 1972 UN Liability Convention define several
forms of space debris related liability: absolute (objectiveþunlimited) liability for the damage caused by a space
object on the ground, fault liability for the damage caused by a
space object to another space object or to a human being in
outer space, and joint liability for damage caused by space
debris resulting from a first collision with another space
object. While there is a framework to hold state or entity
accountable for accidental/purposeful creation of debris and
damages to assets (on Earth or in orbit), the enforcement of
such liability remains a considerable challenge [59].
Nations and companies attempting to adhere to the
IADC's 25-year de-orbit guideline for LEO encounter liability concerns stemming from the 1972 Liability Convention, because this guideline promotes uncontrolled reentry, and as a consequence, potential terrestrial damage.
Article II of the 1972 Liability Convention affirms that “a
launching State shall be absolutely liable to pay compensation for damage caused by its space object on the surface
of the Earth or to aircraft in flight.” Thus, although the 25year de-orbit guideline reduces the potential for on-orbit
collisions with other space objects, this guideline does not
reduce the collateral possibility of on-ground damage
incurred during re-entry, for which operators may be held
liable per the 1972 Liability Convention. Several large
defunct satellites in sun-synchronous orbits have decay
times that are much larger than 25 years. For the operator
this means the ground damage liability concern has been
moved many decades away. The immediate cost and risk
to the operator appear to have been reduced, but only if
ignoring the growing risk of these large debris objects
colliding with other space objects [63]. Fig. 6 demonstrates
the number of cataloged object re-entrees throughout the
history of space operations. This will likely increase with
greater adherence to the IADC guidelines.
Further, previous debris-related events have shown that
most of the political embarrassment from causing damage is
minimal. One example, the re-entry of COSMOS 954 in 1978,
scattered radioactive debris over about an 800 km Section of
Northern Canada due to a malfunction in the boost system
for its nuclear reactor. The cost of clean-up was around $14
million, and the Soviet government only paid $3 million of
that [65]. While it was evident that the debris was from the
USSR, their payment and liability was minimal. However, if
the operator is trying to deorbit their satellite, care must be
taken to avoid the debris hitting populated areas, or causing
Fig. 6. Cataloged objects that have re-entered, replicated from Reference [64].
general damage. Some countries, such as France, are enacting
strict liability if debris causes terrestrial damage after any
attempt of debris mediation or remediation.8 While to date
the direct human harm due to re-entering space debris is
virtually zero [66],9 if falling debris causes a death, it may
significantly alter the discussion about liability, especially in
the effected country. As a result, operators must consider the
risk and political cost of deorbiting an object and the risk of
doing terrestrial damage, versus the risk of leaving the object
in space for many decades. Reference [67] discusses how the
current guidelines, stating an acceptable casualty limit for
random reentry of Ec ¼ 10 4 , is actually hindering development of space debris solutions. While many of the large LEO
debris objects already exceed this casualty risk mark, developing ADR strategies that can guarantee this guideline after
an active mitigation or remediation maneuver will significantly drive up the debris removal technology costs. Thus, if
this guideline prevents cost-effective debris removal technologies to be developed, it will lead to a higher casualty risk
as the existing large LEO objects will reenter, uncontrolled, in
a few decades. Reference [67] proposes a relaxed “interim
provision” guideline that allows for the first generations of
space debris removal technologies to be tested.
Another example is the Chinese 2007 anti-satellite
(ASAT) test which was the most catastrophic fragmentation event recorded. Statements were made by several
space faring nations10 that expressed the concern for such
actions. This ASAT test was performed at a higher altitude
where the debris will remain for decades. The US conducted an ASAT operation of their own11 shortly afterwards. Because this ASAT action was performed at a low
altitude to destroy a hydrazine tank on-board a malfunctioning spacecraft, the atmospheric drag has removed
most of the associated debris. These two events have
greatly raised the awareness of risks and political costs
of performing ASAT operations. However, at the time,
beyond some space debris experts expressing concern,
there was very little popular awareness of the issues, or
political fall-out. However, in the last few years there has
been an increase of popular articles,12 and the movie
Gravity that dramatizes the threats of space debris. If
8
http://tinyurl.com/pdlwsfs
http://tinyurl.com/k7emxdo
http://news.bbc.co.uk/2/hi/asia-pacific/6276543.stm
11
http://tinyurl.com/qxfs4xp
12
http://tinyurl.com/nlbwpao
9
5
6
7
http://www.unoosa.org/oosa/SpaceLaw/liability.html
http://www.unoosa.org/oosa/SORegister/regist.html
http://www.unoosa.org/oosa/SpaceLaw/outerspt.html
10
H. Schaub et al. / Acta Astronautica 113 (2015) 66–79
73
another large debris field generating high altitude ASAT
were performed today, the political fallout and costs to
operators would be higher than with these tests. A high
altitude ASAT test by any country could be both a political
“black eye” and impede further collaborations between the
country performing the test and the rest of the spacefaring community.
Looking at the Iridium–Cosmos collision in 2009, there
were no legal repercussions for the Russians or Iridium
space operators, although Iridium lost a functioning spacecraft. At the time of the collision, the two bodies were not
expected to collide.13 In fact, they were not even among
the most likely space objects to collide that day. Iridium
ended up maneuvering into the Cosmos' flight path, and
this collision has helped increase the communication
between the US Air Force and satellite operators. Possible
conjunctions events are slightly more transparent, and
planned maneuvers can be verified a priori to avoid a
resulting conjunction event. This satellite-to-satellite collision naturally resulted in direct costs to Iridium, but also
indirect costs to users of space, specifically the USAF. Many
operational lessons have been learned since the Iridium–
Cosmos collision. Today's political costs, if an operator
were to maneuver into the path of another object, are
expected to be higher because of the improved tracking
data being provided, and the increased awareness of space
debris hazards.
Next, consider the multi-ton spacecraft Envisat which is
no longer operational, but remains near the popular Atrain obit. As discussed earlier, this sun-synchronous orbit
region has the highest risk of colliding with other debris,
and is in a heavily populated zone of expensive, commercial satellites. The loss of the ESA Envisat satellite in April
2012 has prompted many in the European Union to
consider ways to actively remove this object from orbit
due to its large size and high orbit.14,15 With ESA attempting to follow international guidelines and publicity of this
event in space debris circles, there appears to be a genuine
effort to consider ways to remove this object from orbit
[26,68–70]. Besides considering the technical challenges of
moving Envisat, the political costs would be considerable if
Envisat were to collide with another operating satellite,
and cause a massive debris field in the highly commercial
sun-synchronous orbit regime. This appears to be one of
few current debris concerns where a potential political
black eye is causing some ADR research in Europe. In the
end, even these political costs are not high enough to
currently warrant funding an Envisat ADR mission, but the
pressure is mounting.
35 collisions in the next 100 years. Increased mitigation
adherence will significantly lower this, to about half as
many collisions ( 15).
It can be argued that these studies are not really
representative because they are mean values. Outlier conditions, such as the Iridium–Cosmos collision, will drive the
debris population, not the mean [63]. For example, if there
were two collisions back-to-back, this will cause many more
challenges versus if there is only one collision within 15
years. Mean growth studies are not a good illustration of how
good, or bad, the space debris environment will be.
With the recent Chinese ASAT test and Iridium–Cosmos
collision, the number of new objects created was equivalent to about 16 years of launches [35]. It can be said that
years of successful mitigation can be negated by one
collision [59]. In this way, collisions, and the associated
increase in space debris density in the associated orbit
regime, can be considered a enhancer/multiplier for all
previous costs. This is also the case for indirect and
political costs which are enhanced by the space environment weight factor.
DigitalGlobe created an automated conjunction analysis tool (at cost to the company) that analyzes the JSpOC
conjunction reports. This tool was created only after the
Iridium–Cosmos collision, emphasizing that collisions
increase infrastructure costs. If there are enough conjunction events per day, a dedicated analysis team may be
required, increasing costs to operators further. Further,
more collisions will eventually cause insurance rates to
increase on satellites. The SwissRe report already indicates
that the rates for GEO satellites are being reconsidered to
account for growing space debris [38].
If a collision were to occur in heavily populated orbital
regimes, such as the sun-synchronous orbits or GEO, this
would have even greater effect. A collision in GEO will
distribute debris throughout the entire belt within a day
[72]. As illustrated in Fig. 3, the GEO gravity wells will
causes some of this debris to be focused on these regions,
increasing the local-longitude conjunction likelihood even
further. A collision in the commercially popular sunsynchronous orbits could result in costly direct costs to
locally operating satellites, and make future operation in
such sun-synchronous orbits more challenging. Both of
these narrow orbit regimes are special in that the operators are flying there to take advantage of particularly
favorable orbital physics. These mission types are not
possible by simply moving the GEO satellite to a lower
orbit, or changing the plane of the sun-synchronous
spacecraft.
3.5. Environmental costs
4. Operator responses to space debris
Both ESA and NASA have looked into the predicted
mean number of collisions expected, in LEO, over the next
hundred or two hundred years [1,71]. Without changes to
current-day practices, it is expected that there will be over
This paper considers two reactive and two proactive
operator or policy maker responses to the threat of space
debris. The reactive options are to ignore space debris, or
to track debris and perform collision avoidance maneuvers
if required. The two primary long-term, proactive avenues
are to improve the debris environment either through
stronger mitigation efforts or commencement of active
debris removal (ADR) missions.
13
14
15
http://celestrak.com/events/collision.asp
http://tinyurl.com/6tja2t7
http://tinyurl.com/kq96x5x
74
H. Schaub et al. / Acta Astronautica 113 (2015) 66–79
4.1. No action related to space debris threat
Many LEO satellite operators do not respond to debris
hazards because (a) their spacecraft does not have any
maneuvering capability, or (b) they do not consider the
conjunction information reliable enough to justify the cost of
an avoidance maneuver. Inherent in the later consideration is
the impact of spacecraft shielding. This provides some level of
security to the operator that the satellite could withstand
impingements with very small debris.
The no action option is certainly the most economical
response during a mission, but also the one with a high
risk. There is a growing population of small- and nanosatellite missions that have no propulsion capability. These
missions are often high-risk technically and designed for a
short mission duration. When considering all the risks that
might terminate their operation, the probability of being
hit by debris is not a driving consideration.
Some satellites with maneuvering capabilities still
choose not to respond to debris conjunction threats and
rely on shielding to handle the small debris. Regarding
large debris threats, operators might arrive at this noaction response because they don't have access to conjunction data, feel the uncertainty of the conduction
prediction is so large (often 10's of km or more) that they
cannot justify the cost of an avoidance maneuver, or they
choose to avoid additional maneuvers to retain fuel for
other mission objectives and simply accept the higher
mission risk. As illustrated in Fig. 5, prior to the Iridium–
Cosmos collision this “no-response” decision was the most
common response.
4.2. Dodging space debris
The “debris dodging” response considers moving an
operating spacecraft to avoid another space object, or
changing the flight path of debris to avoid a collision.
Since the Iridium–Cosmos collision many operators'
response to debris threats have evolved. The mediumand large-debris conjunction assessments are more reliable and transparent, and the 2009 event raised the
awareness of debris collision risk. Thus, since 2009, an
increased number of operators is choosing to make orbit
corrections to effectively weave through space debris field.
Conversations with LEO and GEO space operators showed
that these corrections are commonly integrated into the
regular orbit maintenance maneuvers. Thus, typically no
additional fuel is expelled, but the direction of the burn is
slightly adjusted. However, note that these maneuvers are
only possible with respect to tracked debris (10 cm and
larger). About 98% of LEO debris falls into the LNT category,
and thus cannot be dodged. While this dodging strategy
only avoids about 2% of LEO debris, it does help avoid the
large-on-large collisions that are a major source of the
small LNT debris.
Dedicated collision avoidance maneuvers will use up
some of the mission fuel reserves, and are currently very
rare. The conjunction uncertainty would need to be rather
low for such a decision. Or, if the mission is of high value
(commercial satellite, costly science satellite, etc.) the
operator may choose to perform a burn despite large
conjunction uncertainty.
Weaving and dodging about the medium- and largesized debris is currently an affordable option for the space
operators because the various indirect costs, such as
tracking and cataloging the debris, is not currently charged
to the space operators. However, if this should change in
the future, and operators would have to pay for good
tracking data (pushing infrastructure costs onto operators), the cost balance would shift in favor of proactive
debris responses.
A recent development is the concept of Just-in-time
Collision Avoidance (JCA) technologies [33]. Here shortnotice (within days or hours) intercept methods allow a
large debris object to be nudged, thus avoid hitting
another large debris object, or an operating satellite. While
this method can be cost-effective in avoiding an immediate large-on-large fragmentation event, this method only
moves the debris, and does not remove it from orbit and
future conjunction events. However, it could provide
critical near-term large debris protection to a space operator willing to fund such an action.
4.3. Stronger mitigation implementation and/or practices
Mitigation is the process of reducing the likelihood that
a specific object will cause more debris. Unlike the earlier
two actions, it is not a response to an immediate debris
threat. The first, and most widely used, mitigation practice
involves passivation where rocket bodies and satellites
that have reached end-of-life dump fuel, short out batteries, and effectively reduce the amount of on-board
stored energy (they should also have captive mechanism
features). Prior to the ASAT tests and the Iridium–Cosmos
collision, it was post-mission explosions due to stored
energy that caused the most debris growth. Passivization
minimizes potential break-ups in orbit. Most space agencies have their own passivation guidelines, and their
common implementation has greatly helped reduce the
small debris growth.
The second mitigation practice, that is generally not as
widely followed, is to implement post mission disposal.
The most widely referenced general mitigation guidelines
are the United Nation's (UN) IADC and Committee on the
Peaceful Uses of Outer Space (COPUOS) Mitigation Guidelines. The well known LEO 25 year deorbit and GEO
235 km reorbit come from these guidelines. These guidelines are good step in the right direction because they
focus on limiting the generation of future debris. However
the guidelines, again, can be challenging to enforce. Some
countries, such as France and Belgium, have legislation
making the guidelines binding for their space operators.
For most countries the COPUOS guidelines are not binding
and have a recommendatory value.
As discussed previously, it is currently more costeffective in the short-term for companies to create debris
– i.e. not clean up their end of life satellites. Therefore,
with the increasing commercial activities in orbit (both
LEO and GEO), there is little short-term fiscal incentive to
push for better mitigation practices. This enforces the
concept of a global means for enacting and enforcing the
H. Schaub et al. / Acta Astronautica 113 (2015) 66–79
Number of GEO Satellites
28
Achieved End-of-Life
Attempted Re-orbiting
Successful Re-orbiting
24
20
16
12
8
4
97
19
98
19
99
20
00
20
01
20
02
20
03
20
04
20
05
20
06
20
07
20
08
20
09
20
10
20
11
20
12
20
13
0
19
use of mitigation measures on all missions. If instituted
globally, all contractors are equally impacted and will
remain commercially competitive. What this does not
mean is that all mitigation policies need to be the same
for all orbit regimes. For example, GEO and LEO already
have different guidelines. As illustrated in Fig. 2, the LEO
debris distribution is focused in a finite set of orbit
regimes. It is conceivable that refined mitigation guidelines are developed for particular LEO regions.
Jakhu suggests that the only way for these mitigation
methods to be effective is to have binding international
legal agreements that are reflected in the domestic laws of
space-faring nations [59]. This would require international
cooperation across many interests and domestic policy to
be made that could be unpopular within the space community, due to increased regulations. Laws such as The
French Space Operations Act from 2008, while well meaning, could remove competitiveness for French commercial
entities, since the rest of the world is not restricted by
similar laws. The challenge is that, economically, instituting mitigation regulations appears to be an “all or nothing”
effort where the only way for any company to internationally be competitive is for all nations to have similar
regulations.
Current mitigation guidelines have been shown to not
be enough to reduce debris growth in LEO [34] and there is
debate about whether the GEO rules are a reasonable
solution [73]. Further, adherence to the 25 year rule for
LEO is weak [74]. Only 15% of the high debris criticality
index spacecraft (often in high sun-synchronous orbits)
are being maneuvered to have a decay time less than 25
years. In GEO, operators are much better at self-regulating
and appear to be improving with their efforts to comply
with post-mission IADC disposal guidelines. It should be
noted that the fuel cost to reorbit the end-of-life GEO
satellite is only about 11 m/s, much less than the 100's of
m/s required to deorbit LEO satellites. This could explain
the higher compliance rate at GEO. Using re-orbit statistics
compiled from ESA's annual Classification of Geosynchronous Objects reports [47], Fig. 7 illustrates compliance to
the IADC re-orbit guidelines since they were introduced to
the international GEO operator community in 1997 [75].
Fig. 7(a) shows the number of GEO satellites annually that
(1) reached the end of their operational lifespans, (2)
attempted re-orbit to an IADC-compliant disposal orbit,
and (3) successfully achieved the minimum periapsis
altitude increase stipulated in the IADC guidelines. The
margin between the number of assets that reached end-oflife and those that re-orbited into an IADC-compliant
disposal orbit has decreased since 1997, indicative of a
growing international desire to preserve the long-term
utilization of the GEO ring. Fig. 7(b) provides a breakdown
of the compliance data in Fig. 7(a): during 1997–2013,
approximately 50% of GEO satellites that reached end-oflife were repositioned into IADC-compliant disposal orbits
before deactivation, and 30% attempted post-mission disposal, but were unsuccessful in achieving the IADC's
minimum periapsis altitude increase. Interestingly, 53
GEO satellites were abandoned without any re-orbit
attempt during this time frame, the largest contributor
being Russia (33 abandoned). The political costs to Russia
75
Year
Compliant
133 (50.4%)
China
34
IntelSat
8
Japan
15
52
Russia
18
USA
Other
6
5
5
4
4
9
9
22
IADC
Noncompliant
78 (29.5%)
33
33
61
Abandoned
53 (20.1%)
Fig. 7. Compliance to IADC guidelines for post-mission disposal of GEO
satellites [47]. (a) Annual GEO disposal guideline compliance since introduced. (b) Compliance to GEO disposal guidelines by entity for 1997–2013.
contributing 62% of the abandoned GEO satellites over this
time period has been negligible. This illustrates that for the
near-term it is cheaper to generate debris, than try to
follow debris mitigation guidelines.
The natural question arises, how can LEO operators be
encouraged to act more like GEO operators? Debris mitigation is a key element to an economically sustainable
space debris environment. The high GEO operators' compliance rate is facilitated through:
1. Awareness of the debris-related mission risk: The low risk
tolerance of GEO operators, as shown by the lion's
share of the space insurance being at GEO, and the
narrow operating regime about GEO make the operators sensitive to space debris issues.
2. Availability of low-cost debris mitigation solution: It only
requires about 11 m/s of Δv to boost a satellite to a
super-synchrounous disposal orbit.
3. Avoidance of the myriad of legal concerns about causing extremely rare terrestrial damage, as the satellites
do not re-enter the Earth's atmosphere.
In contrasts, many LEO missions are lower-budget and
more risk tolerant. Even for insured commercial satellites,
76
H. Schaub et al. / Acta Astronautica 113 (2015) 66–79
the operators engage in “debris dodging” to avoid trackable LEO debris, often not realizing they are avoiding only
3% of the potentially lethal LEO debris population, and thus
have about the same chance of their mission pre-maturely
terminating due to a collision with a LNT object.
The fuel efforts required to deorbit are significantly
larger, around 100's of m/s for a sun-synchronous spacecraft. Low-cost solutions to deorbit end-of-life satellites is
critical. However, such technologies, such as drag or tether
devices, will result in a passive re-entry. This raises
concerns with about causing terrestrial damage. A LEO
operator may chose to simply abandon a satellite in orbit,
rather than doing an active deorbit and assume the legal
risks. Thus, to encourage LEO operators to comply better
with debris mitigation guidelines, it is key to continue to
educate them on the true current and future debris risk,
while providing access to low-cost and low-liability deorbiting solutions.
Another important question is whether more aggressive rules should be applied for particular orbit regimes
(much shorter than 25 years after end of life – LEO,
reboosting of objects significantly beyond 235 km –
GEO). At the 6th European Conference on Space Debris in
Darmstadt, Germany, McKnight voiced the idea of using
shorter post-mission disposal times such as 10–15 years.
These measures might stabilize the debris population
without needing ADR. Such stronger mitigation guidelines
would increase the mission costs of most space operators,
thus common enforcement would be critical. While there
has been progress on improving mitigation, the slow pace
of acceptance, especially in the countries that are the
largest contributors to debris, makes it appear that the
political and economic costs are (perceived as) currently
more than the cost of increasing debris.
4.4. Remediation
If operators and policy makers cannot be convinced
that mitigation regulations should be adopted, remediation appears to be the only other option for reducing
growth to stabilize the debris density. While the ideal
scenario will have nations participating in mitigation AND
remediation, remediation is attractive because it can have
very significant affects for small numbers of objects
removed from orbit. Further, remediation can be performed readily unilaterally if satellite operators are moving their own debris. This is good because a single country,
or small group of organizations, can directly improve the
entire orbital environment and not necessarily need a
world-wide policy effort.
The problem with unilateral implementation is that the
direct expense of these systems to the organizations
involved will be large. Even more troubling is that some
ADR concepts could be considered a space weapon. Therefore, if a country like the US or Russia were to build an ADR
system, there would be major concern about the use of
this system. The creation of any ADR system would
preferably be a public endeavor accepted by the international community [62].
Because there are 100,000's of centimeter or larger
sized debris objects in LEO, the economic costs to deorbit
large numbers of debris is daunting. A key driver in ADR
technology developments is the economic viability. Otherwise, it can cost more to remove an object from orbit than
it originally cost to put it into orbit. The economic hurdles
to developing ADR solutions are considerable. However,
the system costs can be reduced by considering that many
ADR-related technologies have a broader use. For example,
the autonomous rendezvous and docking GNC sensor
systems required to approach passive space debris, the
touchless actuation systems, the robotic grapplers, are also
being developed for autonomous orbital servicing, asteroid
capture systems, resupply missions, and space asset harvesting systems. By highlighting the broad use of to-bedeveloped ADR technologies, it is feasible to developed the
required technological readiness level without investing
large funds into a singular ADR concept.
It will likely be challenging for a single country to have
space debris researchers convince their policy makers and/
or population that the expense of an ADR system is
worthwhile. However, it could be argued that ADR system
development will provide vital new technologies with
broad mission applications for that country or industry.
This would make them a leader and key resource within
the space-faring community.
One potential concept for funding of an ADR system
(internationally or regionally) is to tax all operators that
generate debris. Thus, there would be a tax associated with
launching and the debris generation potential of the
mission [62]. This would make the responsible parties
for debris creation pay for the clean-up of space, but again,
this cost structure will hurt competition without universal
adoption.
A particular challenge with raising funds to implement
space debris mitigation and remediation is that they do
not return positive feedback [35]. This is because if
mitigation and remediation measures work, there will be
no major collisions. It is only the negative results, a
collision, that are overly obvious. This makes costs associated with prevention less tangible. However, besides the
well-known Iridium–Cosmos collision, there have been
other cases where space debris has disabled a satellite,
such as with Cerise [54]. Tracking such debris-related
failures is critical to convincing policy makers that the
debris models and forecasts are true. However, it is very
challenging to determine if a satellite failed due to a
collision with a small debris object. Satellites fail for many
reasons, including space weather, mechanical failures,
software glitches, etc. Recent satellite failure workshops
are trying to share the histories of satellite failures to gain
the data to track the exact causes.
But, as the perceived risk due to debris increases, direct
costs to operators will increase because active responses
will be chosen, and the indirect costs to all organizations
will also increase due to the associated space situational
awareness demands. Fig. 8 demonstrates this trend, and
considers three scenarios. When it becomes necessary to
enact “stricter mitigation”, due to higher risk levels and/or
government requirements, this will create a ‘level-set’ of
cost to operators that will be difficult to reduce. This is
because the costs for mitigation will likely be needed for
all future operations to stabilize the debris population. If
H. Schaub et al. / Acta Astronautica 113 (2015) 66–79
Short-Term (Reactive)
Long-Term (Proactive)
Late ADR
Implementation
Direct COST
Potential
'equilibrium' of
cost and risk
Debris
Dodging
regimes debris hazard risk to a much higher value. Again,
early development of ADR technologies is needed to be
able to respond to such outlier events.
5. Conclusions
Collis
ion
Early Stricter
Mitigation
No Action
77
Stricter
Mitigation
Enact ADR
Level of Perceived RISK
Fig. 8. Potential direct cost to operators as risk increases, and the trend in
mitigation versus remediation.
the risk increases even more for a particular orbit regime,
the total costs eventually increase enough that “ADR”
becomes cost effective. Of course, the earlier ADR is
implemented, the lower the cost for reducing risk.
McKnight also makes this point in Reference [35].
The following three scenarios are considered. First, in
the “Late ADR Implementation” scenario, assume the
debris hazard risk in an orbit regime is allowed to grow
to the point where even stronger mitigation measures will
not sufficiently shrink it. The only option is to implement
ADR to help remove objects that contribute to large scale
debris growth [1]. The cost of such ADR technologies
would be very high if this decision to engage in ADR is
made with short notice [35]. A more economical approach
would be to develop ADR technologies early on, possibly
for other mission scenarios such as orbital servicing. Next,
the interesting question is, how long will ADR measures be
necessary? At some point ADR has removed enough
objects such that the debris population is stabilized at a
reasonable level. If the more costly ADR operations were to
be suspended, would space operations reach an equilibrium where stricter mitigation is enough to stabilize
the risk?
In the second scenario, consider the case where in an
orbit regime the risk has grown large, but can still be made
stable with stricter mitigation. For example, in sunsynchronous orbits, would a 10–15 year post-mission
disposal time reduce the debris hazard risk without needing ADR? It is envisioned here as well that over time an
operational equilibrium will be achieved.
Finally, consider the best-case scenario where with
early thoughtful actions in an orbit regime an operational
equilibrium with the debris hazard risk has been achieved.
At first glance it may seem this avoids any need to develop
ADR technologies. However, ADR capabilities would still
be required to keep this orbit regime's debris stable. While
the risk is held steady at a lower value, the risk is not zero.
Collisions being unlikely does not imply they are impossible (see Iridium–Cosmos collision probability). A catastrophic collision can still occur, even in a low-risk
environment, whose outcome would jump the local orbit
This paper outlines the complex cost considerations
associated with space debris hazards for the space operators.
When advocating stricter mitigation or active debris removal
solutions, these must be considered to yield effective guidelines for sustainable space operations. Further, operational
equilibria are postulated where the debris mitigation costs
and operational risks are balanced. However, such equilibria's
would not be stable, as even a low probability of collision can
result in an actual collision. For example, the 2007 and 2009
events caused the equivalent of 16 years of debris within only
2 years. With the increasing launch rates and incomplete
mitigation implementations, a philosophical change in space
operations is required.
Acknowledgements
The authors would like to thank Doug Englehardt of
Digital Globe Inc., Brandon Jones at the University of
Colorado, Scott Erwin at AFRL and Chris Kunstadter with
the Aerospace Insurance XL Group for their input.
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